The solventless resin compositions of U.S. Pat. No. 4,603,182 have special merit as electrical insulation materials because of their low viscosity and their stability at elevated temperatures, both of which favor the use of vacuum-pressure impregnation technique in the production of insulated conductors. While those compositions have consequently seen extensive use in insulation applications and, in fact, have qualified for use in service at temperatures as high as 220.degree. C., there persists a well-recognized need for vacuum-pressure impregnatable (VPI) resin compositions for electrical insulating purposes at temperatures up to in the 220.degree.-250.degree. C. range for protracted periods. The numerous attempts to produce such heat stable VPI resins have taken a variety of forms, but all have failed for one reason or another to satisfy the demand. Addition of inorganic oxides and silica, for instance, has proven to be of very little benefit. Thus, lamellar silica enhances thermal stability beyond such other inorganic materials but substantially increases viscosity of VPI resins, detracting from their usefulness.
Various heat resistant polymers such as polyimides, fluoro silicones, polyphenylsulfide and the like are useful as films, molded parts and wire enamels but cannot be used at VPI resins for insulating electrical machinery nor can they be used to impregnate mica paper to make prepregged mica tapes. Liquid enamels made with high temperature polyimide polymers usually contain less than 15% solids and, in addition, a volatile compound is generated during cure. Polyimides, such as KAPTON.sup.T , find uses in heat resistant electrical insulation as films but cannot be used as VPI resins for reasons set out above.
None of the commercially-available heat resistant VPI resins which we have tested is more heat stable than those of U.S. Pat. No. 4,603,182 referenced above.